Multiple phases of myogenesis have been observed in all species. For example, in mammals, embryonic myogenesis is followed by fetal myogenesis, then neonatal myogenesis, finally myogenic repair. Although the basic transcriptional machinery for these different phases of myogenesis is similar and reasonably well understood, very little is known about the basic developmental biology of these different myogenic periods. In particular, it is not clear if there is a direct, lineal relationship between cells that underlie these different epochs. Is there one type of myogenic precursor that sequentially produces different types of muscle, or are there many types of precursors? Do the same signals regulate cell fate choices at each stage? Can we learn something relevant to stem cell biology by studying these different phases of muscle patterning? This application seeks to answer these long term questions, using the powerful combination of cellular embryology and genetics possible in the zebrafish. Inspired by lineage experiments, the earliest slow muscle precursors were proposed to be induced by Hedgehog signaling. After these embryonic slow muscle fibers develop, there are one or more phases of myogenesis in zebrafish, and the later, post-embryonic phase of myogenesis is only partially regulated by Hedgehog signaling. This application aims to use single-cell injections of lineage tracers to identify the precursors to this post-embryonic muscle growth. It also aims to characterize the role of Hedgehog signaling in post-embryonic myogenesis, using a combination of genetics, pharmacology, and experimental embryology. This research will answer questions of fundamental importance to the understanding of how cells develop into muscle fibers. This information is critical to understanding muscle diseases including heart disease and muscular dystrophy. When the signals regulating myogenesis are understood, it will be possible to guide stem cells towards a myogenic fate, to design therapies for devastating human diseases.